The present invention relates to a piston intended primarily for use in refrigeration compressors, but also suitable for other applications.
A significant problem with refrigeration compressors is premature failure due to excessive wear of the wrist pin bearing surfaces. Generally, the piston and connecting rod are connected together by means of a solid or tubular wrist pin which extends through the connecting rod and through aligned openings in the piston skirt. The clearance between the wrist pin and wrist pin openings must be carefully controlled so that an oil film of proper thickness can be maintained. If this thickness should be disrupted, as by excessive wear of the wrist pin holes, localized overheating will occur and failure will ensue.
One of the factors which can lead to excessive and premature wear of the wrist pin bearing surfaces is localized overheating thereof caused by the conduction of heat from the head of the piston. In small compressors, which have a relatively short piston stroke, there is a large reexpansion volume, and the same gas is repeatedly compressed and reexpanded and begins to overheat because it is not pumped out of the cylinder. This heat is transmitted directly to the head of the piston, which is generally made of aluminum or other good thermal conductor, and is conducted along the piston skirt to the wrist pin bearing surfaces. The transfer of heat is enhanced because the wrist pins are normally supported in wrist pin bosses, which are substantially thicker than the remainder of the piston skirt and serve as excellent heat transfer sections.
As the compressor is operated, the temperature of the wrist pin bearing surfaces rises and the viscosity of the oil decreases. At high temperatures, the lubricity of the oil is virtually non-existent, and there is little or no oil film to support the load between the wrist pin and piston on the compression stroke. This results in metal to metal contact and the wrist pin will wear an eliptical hole in the wrist pin boss. Once there is a clearance of several thousandths of an inch, the piston assembly will fail in a matter of a few hours.
A very severe test of a compressor was run wherein the compressor was operated at a very high compression ratio. This produces a large reexpansion volume which results in high temperatures being developed at the head of the piston. The compressor was run for about two hundred hours and oil samples were taken from the compressor to observe any change in color. It was observed that the oil began to discolor at about one hundred and fifty hours indicating that the metal had begun to oxidize, and at two hundred hours the compressor was nearly nonfunctional. Upon examination, the wrist pin bosses were worn into an eliptical shape to such a degree that the compression ratio could not be maintained.
With the heat barrier of the present invention, however, the compressor survived for five hundred hours of operation. The oil remained substantially clean throughout this time, and when the bearing surfaces were analyzed following the test, they evidenced no undue wear.
The partial blockage of heat from the head of the piston to the wrist pin bearing surfaces is accomplished by suitably shaped and located openings extending through the wrist pin bosses. Although prior art pistons have been provided with openings in the piston skirt between the head and the wrist pin holes, they are not shaped or positioned such that they serve as thermal barriers. U.S. Pat. No. 2,514,016, for example, discloses a piston having oblong holes cut in the piston skirt near the head of the piston and oriented such that their longitudinal axes extend in the axial direction of the piston. Because the openings are oriented in this manner and located so close to the piston head, heat is able to flow around the openings and nearly directly to the wrist pin holes. Although these openings assist in enabling lubricant to flow from one side of the piston skirt to the other, they do not serve as a thermal barrier.
In U.S. Pat. No. 1,779,555, a plurality of circumferentially spaced oil passages are provided through the piston skirt between the wrist pin and head of the piston. The passages are so small, however, that their effect in blocking thermal conduction to the wrist pin bearing surfaces is negligible.